The present invention relates generally to the photovoltaic art and, more particularly, to a thin film photovoltaic module having transparent front and back electrode arrangements.
Photovoltaic cells of the prior art have typically been opaque, extending from a first, light-incident surface having a transparent conductive electrode to a second, back surface covered by a metallic electrode film. Metallic back electrodes have been favored because they are both conductive and reflective, enhancing collection efficiency of the device and reflecting unused light back through the semiconductive layer for use in generating electricity.
A significant problem associated with photovoltaic panels is the space required to locate them in full view of the sun. If an opaque module casts a shadow on another module for part or all of a day, the shaded area of the second module is effectively removed from the current generating system.
One approach to solving the problem of module size and bulk is to form stacked modules in which one array of cells is positioned over another array of cells so that light passing unabsorbed through the upper array is absorbed by the lower array. U.S. Pat. No. 4,461,922 discloses a stacked arrangement in which an upper thin film panel has a pair of transparent electrodes for passing unused light to a lower panel for photovoltaic purposes. However, such modules are more expensive to manufacture than single panels and are opaque to incident light.
A "bifacial" array able to receive solar energy at opposite surfaces thereof is disclosed in Cuevas, et al., "50 Percent More Output Power From An Albedo-Collecting Flat Panel Using Bifacial Solar Cells", Solar Energy, 29-5, pages 419-420 (1982). The array disclosed therein comprises a plurality of p-type crystalline substrates with diffused regions on opposite major surfaces thereof. Each substrate forms a pair of independent solar cells able to receive light from opposite directions. The cells are made according to costly crystalline silicon technology, including the use of grid-type contacts, and absorb all radiation that reaches the semiconductive material from either direction.
Thin film cells having two transparent contacts are described in Konagai, et al., "The Effect of Residual Impurity B or P on Photovoltaic Properties of Amorphous Silicon Solar Cells", 16th IEEE Photovoltaic Specialists Conference, San Diego, Sept. 27-30, 1982, pp. 1321-1326, but only as an experimental expedient for examining the generation of carriers by independent illumination of the cell either through its p-layer or its n-layer. The article does not propose a practical use for a transparent cell and the two sides of the cells are not illuminated simultaneously.
Therefore, it is desirable in many applications to provide an inexpensive solar module which alleviates problems associated with cell positioning and makes better use of light over the solar spectrum.